The present invention relates generally to optical communications, and, in particular, to an arrangement for dynamic passband shape compensation of optical signals. The arrangement can advantageously control the curvature, tilt, and attenuation of the passband shape, in order to reduce the signal distortion caused by other filters in an optical communication network, and can be used in connection with an integrated multiplexer/demultiplexer.
As the spectral efficiency of wavelength-division multiplexed (WDM) networks increases, the passband widths of the filters that combine and separate the channels approach the signal bandwidth, making the requirements on the passband shapes more stringent. Transmission lines with many add-drop nodes are especially problematic, since small deviations from the ideal passband shape can accumulate to give significant signal distortion. Also, with the use of optical cross connects, the filters that each channel passes through can change. Thus it would be useful to have a compensator with an adjustable passband shape for each channel that can be used in the multiplexing or demultiplexing stage to correct the signal distortions. It would also be advantageous to have a compensator that can control the curvature of a single passband in a relatively rapid manner, instead of the slow acting filter proposed by Sato, Kaneko, and Horiguchi in a paper entitled xe2x80x9cNovel method for controlling passband flatness in optical transparent networks with cascaded AWGs,xe2x80x9d European Conference on Optical Communication, pp. 166-167, 1999.
In accordance with the present invention, the curvature, tilt, and attenuation of the passband of an optical signal is dynamically controlled by an integrated compensator that is advantageously electrically operated. The compensator arrangement can be replicated, and used to independently and dynamically control the passbands of multiple optical signals having different wavelengths, for example in a multiplexing and/or demultiplexing arrangement.
Each compensator includes a xe2x80x9c50/50xe2x80x9d splitter arranged to divide an optical signal into first and second copies. One copy is applied to a first variable optical attenuator (VOA) via a tunable phase shifter, while the other copy is applied to a second VOA directly. Alternatively, both copies can have tunable phase shifters or neither can have a tunable phase shifter but one or both of the VOAs can provide a phase shift. The outputs of the first and second VOA""s are then combined, for example in a planar waveguide grating. In the output on the other side of the grating, the two copies interfere. When the copy phase shift is zero, the net passband is Gaussian; when the copy phase shift magnitude is xcfx80/2, the net passband is flat; and when the copy phase shift magnitude is between xcfx80/2 and xcfx80, the net passband has a dip in the middle. Thus, the tunable phase shifter controls the passband curvature, and the VOA""s control the passband tilt and attenuation level.
The compensator arrangement can be replicated and used to filter a plurality of individual optical signals having different wavelengths. Thus, the arrangement is particularly useful in connection with a wavelength division multiplexing (WDM) optical communication system.